A high frequency power detector and a power regulation method of the general type mentioned above, forming the field of this invention, is disclosed in U.S. Pat. No. 6,265,939 (Wan et al.).
This Patent discloses a power regulation method in which the power is detected and used as an input signal for the power regulating circuit. In the scope of such a power regulation, it is desirable that the input signal varies linearly with the power detected in dBm.
As is generally known, the dimensionless power designation “dBm” is defined as the ten-fold common or base ten logarithm of the power in mW normalized to a reference power of 1 mW. In other words, “dBm” refers to decibels above 1 milliwatt, and defines a measure of power equal to ten times the common logarithm of a ratio of a given power relative to 0.001 watt. Thus, for example, a power of 20 mW can also be designated or identified by the value 13 dBm. Thus, the term “dBm linear high frequency power detector” is understood to mean a detector of which the output signal varies linearly with respect to the power of the high frequency (HF) oscillation which is scaled or designated in dBm.
The high frequency power detector known from the U.S. Pat. No. 6,265,939 comprises a rectifying diode (D1), which produces a DC voltage that is dependent on the high frequency voltage coupled to the rectifying diode through a capacitor. According to the Patent, the DC voltage varies linearly with respect to the HF-power in dBm, at lower powers. However, at higher power levels, a non-linearity in this relationship arises, which must be compensated for. To achieve this, the Patent provides additional circuit stages which add additional incremental loads into the signal processing at various increased HF-power levels. In this regard, the additive incremental loads must be dimensioned so that the output signal of the HF-power detector varies linearly with the HF-power output in dBm. The further circuit parts or stages comprise a network of a DC voltage source and several passive elements, particularly six further diodes (D2 to D7), resistors (R2 to R12), and further capacitors.
The known circuit according to U.S. Pat. No. 6,265,939 is disadvantageous because it requires an active voltage source and a relatively large number of passive components. For this reason, the known circuit can only be realized with a relatively high production effort and expense. Moreover, the great number of necessary components requires a relatively large surface area of the circuit, which is especially disadvantageous in connection with an integration of such a circuit on a single chip.
High frequency power detectors based on diodes generally provide an output voltage, which, depending on the magnitude of the coupled-in high frequency voltage, varies quadratically or linearly with respect to the coupled-in high frequency voltage. Since the power of the electrical high frequency oscillation determines the effective value of the coupled-in high frequency voltage, accordingly the output voltage of the high frequency power detector also varies quadratically or linearly when it is plotted over the power in mW. Since a transformation of the representation of the power values in dBm rather than mW logarithmically compresses the power scale, a representation of the output voltage over the power in dBm will exhibit an exponential characteristic. That is true for both the original linear characteristic as well as the original quadratic characteristic with respect to the power in mW.
In addition to the circuit construction according to the above mentioned U.S. Pat. No. 6,265,939, there are other known high frequency voltage-linear power detectors based on diodes, of which the output voltages must be linearized by at least one external stage. Such a linearizing stage may, for example, be embodied as a logarithmic (or logarithm-producing) operational amplifier, which requires a negative voltage, or as a microprocessor. Furthermore, a calibration process as well as a temperature compensation must also be carried out for processing and compensating the output.